JPH1012951A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a constant gain of a signal light of each wavelength by carrying out weighting with an attenuation factor having predetermined wavelength dependence so as to monitor the power of a wavelength multiplexing signal light, and controlling the excitation light power in response to an output. SOLUTION: A wavelength multiplexing signal light inputted from an input terminal 1 is inputted via an optical coupler 3 and an optical isolator 4-1 to a multiplexer 5. The multiplexer 5 multiplexes an excitation light outputted from an excitation light source 6 and the multiplexing wavelength signal, and inputs the multiplexed light and signal to a rare earth-added fiber 1. An output light of the rare earth-doped fiber 7 is outputted from an output terminal 2 via an optical isolator 4-2. A light partly branched by the optical coupler 3 is inputted to an optical weighting circuit 11. The optical weighting circuit 11 provides the input signal light with an attenuation quantity which is in inverse proportion to the gain of the rare earth-doped fiber 7. The excitation light source 6 is controlled via a control circuit 12 and a light-receiving circuit 8 so that a pre-set excitation power is obtained with respect to the full power of the weighted wavelength multiplexing signal light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber comprising a plurality of optical fiber transmission lines for multiplexing and transmitting a plurality of optical signals having different wavelengths and an optical amplifier for compensating the loss thereof, which are alternately connected in multiple stages. The present invention relates to an optical amplifier used in an amplification relay wavelength division multiplex communication system.

[0002]

2. Description of the Related Art In an optical amplification repeater wavelength division multiplex communication system,
The following problems occur in the optical fiber and the optical amplifier, which are transmission paths, respectively. In the optical fiber, when the signal light power becomes large, nonlinear phenomena such as four-wave mixing and self-phase modulation occur, which causes signal deterioration. In the optical amplifier, when the input signal light power becomes small, spontaneous emission light (ASE light) generated in the optical amplifier becomes large with respect to the signal light, causing signal degradation.

When transmitting wavelength-division multiplexed signal light in such an optical amplification repeater wavelength division multiplexing communication system, if the gain of the optical amplifier has wavelength dependence, the level of the signal light of each wavelength is increased each time the light passes through the optical amplifier. Deviation is accumulated, and after passing through a plurality of optical amplifiers, the level deviation cannot be ignored. As a result, the signal light power increases at a wavelength with a large gain, causing a non-linear phenomenon in the optical fiber.At a wavelength with a small gain, the signal light power decreases and the effect of spontaneous emission increases, and signal degradation also increases. Enlarge. In order to suppress these deteriorations, the output signal light power must be kept constant with respect to variations in the input signal light power, changes in the loss of the optical fiber transmission lines, and changes in the number of wavelength multiplexes caused by the difference in the loss of each optical fiber transmission line. In addition, it is necessary to make the wavelength dependence (gain deviation) of the gain of the optical amplifier small and constant.

FIG. 12 shows a configuration example of a conventional optical amplifier. In the figure, the signal light input from the input terminal 1 is
The signal is input to the multiplexer 5 via the optical coupler 3-1 and the optical isolator 4-1. The multiplexer 5 multiplexes the pump light and the signal light output from the pump light source 6 and inputs the multiplexed light to the rare earth-doped fiber 7. The output light of the rare earth-doped fiber 7 is output from the output terminal 2 via the optical isolator 4-2 and the optical coupler 3-2. The lights partially branched by the optical couplers 3-1 and 3-2 are input to the light receiving circuits 8-1 and 8-2 and converted into electric signals. This electric signal indicates a voltage proportional to the total input signal light power and the total output signal light power. The logarithmic amplifier 9 inputs the outputs of the light receiving circuits 8-1 and 8-2,
Outputs the logarithmic difference. The control circuit 10 controls the output power of the pump light source 6 so that the output of the logarithmic amplifier 9 is always constant.

Here, the relationship between the gain deviation and the gain of the rare-earth-doped fiber amplifier will be described. In a silica-based rare earth-doped fiber amplifier, the logarithmic gain G (λ) of signal light of wavelength λ is represented by σ _e (λ), σ _a (λ), emission cross section and absorption cross section at wavelength λ, and rare earth ions. Where ρ is the density, Γ is the confinement coefficient, N _{2 is the} normalized ion density excited at the laser upper level in the energy level of the rare earth ion, and its average value in the longitudinal direction of the rare earth-doped fiber (hereinafter referred to as the average upper level ion density). <N _2> a) that, when the fiber length of the rare-earth-doped fiber and L, G (λ) = { σ e (λ) + σ a (λ)} ρΓ <N 2> L-σ a (λ) ρΓL ( It is represented by 1).

[0006] The upper-level ion density <N ₂ > indicating the excited state of the rare-earth-doped fiber is represented by the input pump light power p _p , the total input signal light power p _s, and the input end of the rare-earth doped fiber as a reference. Let z be the position in the fiber longitudinal direction when

[0007]

(Equation 1)

## EQU1 ## In equation (1), σ _e (λ), σ _a (λ),
Since ρ and Γ are coefficients uniquely determined by the optical fiber, when the fiber length L is fixed, the logarithmic gain G (λ) is uniquely determined only by the upper-level ion density <N ₂ >. Next, monitoring and control of gain in the configuration of FIG. 12 will be described.

The input signal light power p _si of the wavelength λ _i is obtained by using the total input signal light power p _{s total} of the wavelength multiplexed signal light and the deviation a _i of the signal light power of each wavelength as p _si = a _i · p _{s Total} (3). Here, assuming that the number of signals of the wavelength multiplexed signal light is n,

[0010]

(Equation 2)

There is the following relationship. Further, the total output signal light power p _{s out} of the wavelength multiplexed signal light is obtained by using the logarithmic gain G (λ _i ) of the signal light having the wavelength λ _i.

[0012]

(Equation 3)

## EQU1 ## 12, the total input signal light power p _{s total} and the total output signal light power p
_{Since s out} is monitored by the light receiving circuits 8-1 and 8-2 and the gain is obtained from the logarithmic difference between them, the logarithmic gain G _total for the wavelength multiplexed signal light is:

[0014]

(Equation 4)

## EQU1 ## From the equation (1), the logarithmic gain G (λ _i ) of the signal light having the wavelength λ _i is expressed by the upper level ion density <N ₂
> Is uniquely determined. Therefore, the deviation a of the signal light power of each wavelength as in the loss fluctuation of the optical fiber.
_{If i} is constant and the total input signal power changes, the equation
From (1) and (6), the gain G _{total of the} entire WDM signal light is also <N
₂ > uniquely determined by Conversely, if G _total is _kept constant, <N ₂ > will be kept constant, and the logarithmic gain G (λ _i ) of the signal light of each wavelength, that is, the gain deviation will also be kept constant.

As described above, the control circuit 10 includes the light receiving circuit 8
The gain G _total of the entire wavelength multiplexed signal light is obtained from the logarithmic difference between the output voltages −1 and 8-2, and the pump light power of the pump light source 6 is controlled so that the gain G _total becomes constant. Thereby, even when the total input signal light power changes, the gain and the gain deviation of the optical amplifier can be controlled to be constant. Next, a case where the number of WDM signals changes due to a failure of the transmission light source or the like will be described.

If the signal light source having the wavelength λ _j fails in the wavelength division multiplexed signal light, the gain G _total 'of the entire wavelength division multiplexed signal light is obtained.
Is from equation (6)

[0018]

(Equation 5)

The following relationship is obtained. Here, even when the upper-level ion density <N ₂ > is constant, that is, when the gain at each wavelength is constant, G _total ′ and G _total are G
It matches only when (λ _i ) = G _total and does not match when there is another gain deviation. In the configuration of FIG. 12, the pumping light power is controlled so as to keep the gain G _total of the entire wavelength multiplexed signal light constant. Therefore, if there is a gain deviation, <N ₂ > changes depending on the number of wavelength multiplexed signals. Cannot be kept constant at the wavelength.

FIG. 13 shows an example of calculation of gain for each wavelength when wavelength-division multiplexed signal light is input to the optical amplifier shown in FIG. The optical amplifier was pumped at 980 nm, the pump light power was 50 mW, and the rare earth-doped fiber was an aluminum-codoped erbium-doped fiber of a silica base material. The wavelengths λ _{1 to} λ ₈ of the wavelength multiplexed signal light are 4 nm from 1532 nm to 1560 nm.
It was an interval. The signal light power per wave was -14 dBm. In the example shown here, it can be seen that the maximum and minimum gain deviation is about 3 dB.

[0021]

Even if there is a gain deviation as shown in FIG. 13, when the gain of the signal of each wavelength, that is, the gain deviation does not change, the transmission of the characteristic opposite to the gain deviation is performed. The gain deviation can be compensated by an optical gain equalizer having a ratio, for example, a fiber grating. For this reason, a change in the gain deviation when the input signal light power and the number of wavelength multiplexes change, that is, a change in the gain of each wavelength becomes a problem. Hereinafter, a change in gain of the optical amplifier having the configuration shown in FIG. 12 will be described.

FIG. 14 shows a change in the gain at each wavelength when the number of wavelength multiplexes changes in the optical amplifier having the gain characteristic shown in FIG. Here, as shown in FIG. 12, when the gain of the entire wavelength multiplexed signal light obtained from the logarithmic difference between the total input signal light power and the total output signal light power is kept constant, the wavelength multiplexing number changes. 5 shows a change in gain of signal light of each wavelength.

In the figure, the squares indicate the change in the gain of the remaining signal light when the input of the signal light of wavelengths λ ₇ , λ ₆ , and λ ₁ is eliminated in the order of the gain in the wavelength multiplexed signal light.
The circles indicate wavelength λ in the order of gain from wavelength-multiplexed signal light.
The change in the gain of the remaining signal light when the input of the signal light of ₂ , λ ₃ and λ ₄ is eliminated is shown. When the signal light with a large gain is reduced, the gain of the remaining signal light is increased by about 0.5 dB, and when the signal light with a small gain is reduced, the gain of the remaining signal light is reduced by about 0.7 dB. I understand.

As described above, in the conventional configuration in which the gain of the entire wavelength multiplexed signal light is controlled to be constant, the entire signal light of the wavelength multiplexed signal light is changed like the loss change of the optical fiber which is the transmission line. When the input power changes uniformly, the gain of each wavelength does not change. However, if there is a gain deviation and the number of wavelength multiplexes changes, the gain of each wavelength changes. Therefore, in an optical amplification repeater wavelength division multiplexing communication system configured by connecting optical amplifiers in multiple stages, even if the gain deviation per unit is about 0.5 dB, the change in gain is accumulated due to multiple repeaters. A large change in signal light power causes a problem.

According to the present invention, the gain of the signal light of each wavelength is controlled by both the change in the total input power of the wavelength multiplexed signal light due to the change in the loss of the optical fiber and the change in the number of wavelength multiplexing due to the failure of the transmission light source. An object of the present invention is to provide an optical amplifier that can be controlled to be constant.

[0026]

SUMMARY OF THE INVENTION An optical amplifier according to the present invention includes monitoring means for weighting the input / output power of a wavelength division multiplexed signal light with an attenuation factor having a predetermined wavelength dependency, and according to the output of the monitoring means. Control the excitation light power. In this monitoring means, weighting is applied so as to cancel the effect of gain deviation on the wavelength multiplexed signal light. As a result, it is possible to obtain the input / output power of the wavelength multiplexed signal light without the influence of the gain deviation, and to keep the gain of the signal light of each wavelength constant with respect to both the change of the total input power and the change of the number of wavelength multiplexing. Can be controlled.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment-Claim 2) FIG. 1 shows a first embodiment of the optical amplifier of the present invention. In the figure, a wavelength multiplexed signal light input from an input terminal 1 is input to a multiplexer 5 via an optical coupler 3 and an optical isolator 4-1. Multiplexer 5
Multiplexes the pump light output from the pump light source 6 and the wavelength multiplexed signal light and inputs the multiplexed light to the rare-earth-doped fiber 7. The output light of the rare-earth-doped fiber 7 is output from the output terminal 2 via the optical isolator 4-2. The light partially branched by the optical coupler 3 is input to an optical weighting circuit 11 that weights the optical power of each wavelength at an attenuation rate having a predetermined wavelength dependency. The output of the light weighting circuit 11 is input to the light receiving circuit 8 and is converted into an electric signal. The control circuit 12 controls the excitation light source 6 so that the output of the light receiving circuit 8 has a predetermined excitation light power.

The feature of this embodiment is that the optical weighting circuit 11
In the above, the pumping light source 6 is provided with an attenuation amount that is inversely proportional to the gain of the rare-earth-doped fiber 7 to the input signal light, and the pumping light power is set to a preset pumping light power with respect to the total power of the weighted wavelength multiplexed signal light. In control. Thus, the gain can be kept constant with respect to a change in the number of wavelength multiplexes. Hereinafter, the operation principle of the present embodiment will be described.

The logarithmic gain of the signal light of wavelength λ _i in the rare earth doped fiber 7 is G (λ _i ), the input power is p _si , the logarithmic gain of the pump light is G _p , and the input power of the pump light of wavelength λ _p is p. and _p, the relaxation time from the top of the rare earth ion level to ground level tau, a rare-earth-doped cross-sectional area of the rare earth-doped fiber 7 when the a, the signal light power p of the pumping light power p _p and the wavelength lambda _i The relationship of _si is that if we ignore the influence of ASE light,

[0030]

(Equation 6)

Can be expressed as h is Planck's constant,
c is the speed of light. Further, using equation (8), the pump light power _pp becomes

[0032]

(Equation 7)

Can be expressed as follows. Here, the average upper level ion density at the time of realizing the gain characteristic of the control target is <N ₂
> ₀ , the pump light power p _p0 required to realize the gain characteristic is given by the following equation (9).

[0034]

(Equation 8)

Can be expressed as follows. G _i0 is the gain of the signal light having the wavelength λ _i when the target gain characteristic is realized, that is, when the upper-level ion density is <N ₂ > ₀ . G _p0 is the gain of the pump light when the upper-level ion density is <N ₂ > ₀ . Here, the attenuation characteristic of the signal light of the wavelength λ _i in the optical weighting circuit 11 is inversely proportional to the gain characteristic of the target rare-earth-doped fiber 7, that is, the transmittance T (λ _i ) of the optical weighting circuit 11 is changed by the gain. Give in proportion to the characteristics. That is, T (λ _i ) = aexp [G _i0 ] (11). Here, a is a proportional constant. Such an optical weighting circuit 11 can be composed of, for example, a fiber grating. Further, an optical lattice filter obtained by cascading Mach-Zehnder interferometers and changing the coupling efficiency and the phase difference between two optical paths of the Mach-Zehnder interferometer can be used.

The output pin _mon of the light receiving circuit 8 is obtained by using the transmittance T (λ _i ) of the light weighting circuit 11 expressed by the equation (11).

[0037]

(Equation 9)

Can be expressed as follows. In the control circuit 12,
For values of the output pin _mon of the light receiving circuit 8, it is set in advance pumping light power _{p p p p = α + βpin} mon (13). Here, λ _S is the average wavelength of the wavelength-division multiplexed signal light, and the coefficients α and β in Equation (13) are

[0039]

(Equation 10)

It is assumed that Usually, λ _S ≒ λ _i , exp (G _i0 ) −1
Since ≒ can be approximated by exp (G _i0), for implementing the pumping light power p _p corresponding to the output of the light receiving circuit 8 of the formula (13), the gain characteristics of the target represented by the formula (10) It can be understood that the pump light power p _p0 required for the above is substantially the same. Therefore, for values of the output pin _mon of the light receiving circuit 8, by controlling the pumping light source 6 so that the pumping light power p _p of the formula (13), it can be seen that control the gain characteristics constant . Also, in this case, when Expressions (10) and (13) are compared, it is understood that the gain characteristics can be controlled to be constant even when the number of wavelength multiplexes changes.

FIG. 2 shows a change in gain at each wavelength when the number of wavelength multiplexes changes. In the figure, the crosses indicate changes in the gain of the remaining signal light when the input of the signal lights of wavelengths λ ₇ , λ ₆ , λ ₁ is eliminated in the order of the gain in the wavelength multiplexed signal light. The mark “◇” indicates a change in the gain of the remaining signal light when the input of the signal lights of the wavelengths λ ₂ , λ ₃ , and λ ₄ is eliminated in the order of smaller gain among the wavelength multiplexed signal lights. For comparison, the gain change of each wavelength in the conventional configuration shown in FIG. As shown in the figure, it can be seen that the gain change which was conventionally ± 0.5 dB or more was sufficiently suppressed. That is, when the wavelength multiplexed signal light is weighted by the optical weighting circuit 11 and the output thereof is controlled so as to have a predetermined pumping light power, the gain of each wavelength does not change and the gain is kept constant. It turns out that it can be controlled.

(Second Embodiment-Claim 3) FIG. 3 shows a second embodiment of the optical amplifier of the present invention. In the figure,
The wavelength multiplexed signal light input from the input terminal 1 is input to the multiplexer 5 via the optical coupler 3-1 and the optical isolator 4-1. The multiplexer 5 multiplexes the pump light output from the pump light source 6 and the wavelength multiplexed signal light and inputs the multiplexed light to the rare-earth-doped fiber 7. The output light of the rare earth-doped fiber 7 is output from the output terminal 2 via the optical isolator 4-2 and the optical coupler 3-2. The light partially branched by the optical coupler 3-1 is input to an optical weighting circuit 11 for weighting the optical power of each wavelength with an attenuation factor having a predetermined wavelength dependency. The output of the light weighting circuit 11 is input to the light receiving circuit 8-1 and is converted into an electric signal. On the other hand, the light partially branched by the optical coupler 3-2 is input to the light receiving circuit 8-2 and is converted into an electric signal. The logarithmic amplifier 9 receives the outputs of the light receiving circuits 8-1 and 8-2 and outputs the difference between the logarithms. The control circuit 10 controls the output power of the pump light source 6 so that the output of the logarithmic amplifier 9 is always constant.

The feature of this embodiment is that the optical weighting circuit 11
, The attenuation of the input signal light is inversely proportional to the gain of the rare-earth-doped fiber 7, and the pumping light power is adjusted so that the logarithmic difference between the weighted total power of the input signal light and the total power of the output signal light is constant. Is where you control. In the conventional configuration, the gain of the entire wavelength multiplexed signal light is controlled to be constant even if the number of wavelength multiplexes changes, and the gain of each wavelength changes. On the other hand, in the configuration of the present embodiment, since the gain of each wavelength of the input signal light is compensated and the total power is monitored, the gain can be kept constant with respect to the change in the number of multiplexed wavelengths. Hereinafter, the operation principle of the present embodiment will be described.

The output pout _mon of the light receiving circuit 8-2 has a wavelength λ
_Assuming that the logarithmic gain G (λ _i ) of the signal light of _i , the pump light power p _si , and the proportionality constant b,

[0045]

[Equation 11]

Is represented by Here, the average upper level ion density at the time of realizing the gain characteristic of the control target is <N ₂ > ₀ , the gain of the signal light of wavelength λ _{i at} the time of realizing the target gain characteristic is G _i0 , The transmittance of the weighting circuit 11 is given by the equation (1
If expressed by 1), the output pin _{mon of the} light receiving circuit 8-1
Is

[0047]

(Equation 12)

## EQU5 ## In the logarithmic amplifier 9, the expression (1
When the logarithmic difference between pout _mon expressed by 6) and pin _mon expressed by equation (17) is obtained, the output is

[0049]

(Equation 13)

Is as follows. Here, only when the average upper-level ion density is <N ₂ > ₀ , the logarithmic gain G of the signal light of wavelength λ _i
(λ _i ) is equal to G _i0, and it can be seen that equation (18) becomes b / a regardless of the number of multiplexed wavelengths and the input power. Therefore, when the control circuit 10 controls the output power of the excitation light source 6 so that the output of the logarithmic amplifier 9 becomes b / a, the average upper-level ion density can be set to <N ₂ > _0, and the target gain It can be controlled to have characteristics.

FIG. 4 shows a change in gain at each wavelength when the number of wavelength multiplexes changes. In the figure, the crosses indicate changes in the gain of the remaining signal light when the input of the signal lights of wavelengths λ ₇ , λ ₆ , λ ₁ is eliminated in the order of the gain in the wavelength multiplexed signal light. The mark “◇” indicates a change in the gain of the remaining signal light when the input of the signal lights of the wavelengths λ ₂ , λ ₃ , and λ ₄ is eliminated in the order of smaller gain among the wavelength multiplexed signal lights. For comparison, the gain change of each wavelength in the conventional configuration shown in FIG. As shown in the figure, it can be seen that the gain change which was conventionally ± 0.5 dB or more was sufficiently suppressed. That is, when the input signal light is weighted by the optical weighting circuit 11 and the pumping light power is controlled so that the logarithmic difference between the input and output powers becomes constant, the change in the gain of each wavelength with respect to the change in the number of wavelength multiplexing is obtained. It can be seen that the gain can be controlled to be constant.

(Third Embodiment-Claim 4) FIG. 5 shows a third embodiment of the optical amplifier of the present invention. In the figure,
The wavelength multiplexed signal light input from the input terminal 1 is input to the multiplexer 5 via the optical coupler 3-1 and the optical isolator 4-1. The multiplexer 5 multiplexes the pump light output from the pump light source 6 and the wavelength multiplexed signal light and inputs the multiplexed light to the rare-earth-doped fiber 7. The output light of the rare earth-doped fiber 7 is output from the output terminal 2 via the optical isolator 4-2 and the optical coupler 3-2. The light partially branched by the optical coupler 3-1 is input to the light receiving circuit 8-1, and is converted into an electric signal. On the other hand, the light partially branched by the optical coupler 3-2 is input to an optical weighting circuit 11 for weighting the optical power of each wavelength at an attenuation rate having a predetermined wavelength dependency. The output of the light weighting circuit 11 is input to the light receiving circuit 8-2 and is converted into an electric signal.
The logarithmic amplifier 9 receives the outputs of the light receiving circuits 8-1 and 8-2 and outputs the difference between the logarithms. The control circuit 10 controls the output power of the pump light source 6 so that the output of the logarithmic amplifier 9 is always constant.

The present embodiment is characterized in that the optical weighting circuit 11 for inputting the output signal light includes the rare-earth-doped fiber 7
Of the pumping light is controlled so that the logarithmic difference between the total power of the input signal light and the total power of the weighted output signal light becomes constant.
In this embodiment, the same principle as that of the second embodiment is used, and the gain of each wavelength of the output signal light is compensated and the total power is monitored, so that the gain is kept constant with respect to the change in the number of wavelength multiplexing. Can be. Hereinafter, the operation principle of the present embodiment will be described.

The output pin _mon of the light receiving circuit 8-1 is

[0055]

[Equation 14]

## EQU5 ## Further, the transmittance T (λ _i ) of the signal light of the wavelength λ _i of the optical weighting circuit 11 is

[0057]

(Equation 15)

As described above, the gain is set to be inversely proportional to the gain characteristic of the target rare earth-doped fiber. The output pout _mon of the light receiving circuit 8-2 is obtained by using the transmittance represented by the equation (20).

[0059]

(Equation 16)

Is expressed as follows. The logarithmic amplifier 9 obtains the logarithmic difference between pout _mon expressed by the equation (21) and pin _mon expressed by the equation (19).

[0061]

[Equation 17]

Is obtained. Here, only when the average upper-level ion density is <N ₂ > ₀ , the logarithmic gain G of the signal light of wavelength λ _i
It can be seen that (λ _i ) is equal to G _i0, and equation (22) becomes a / b regardless of the number of wavelength multiplexing and input power. Therefore, when the control circuit 10 controls the output power of the excitation light source 6 so that the output of the logarithmic amplifier 9 becomes a / b, the average upper-level ion density can be set to <N ₂ > _0, and the target gain It can be controlled to have characteristics.

(Fourth Embodiment-Claim 4) FIG. 6 shows a fourth embodiment of the optical amplifier of the present invention. This embodiment has a configuration in which the optical weighting circuit 11 in the third embodiment is arranged between the optical isolator 4-2 and the optical coupler 3-2. Other configurations are the same as those of the third embodiment. The operation principle is the same as that of the third embodiment, and the pumping light power is controlled so that the logarithmic difference between the total power of the input signal light and the total power of the weighted output signal light is constant. The feature of the present embodiment is that the gain of the entire optical amplifier is reduced by the insertion loss of the optical weighting circuit 11, but the attenuation is proportional to the gain of each wavelength to the output signal light. The gain deviation of the wavelength-multiplexed signal light can be compensated at the same time.

(Fifth Embodiment-Claims 2 and 5) FIG.
Shows a fifth embodiment of the optical amplifier of the present invention. This embodiment is applied to the first embodiment (FIG. 1) corresponding to claim 2, and has a configuration in which an optical variable attenuator 13 is disposed between the optical isolator 4-2 and the output terminal 2. Yes, it is applied to fluoride-based rare earth doped fiber optical amplifiers.

In the figure, the wavelength multiplexed signal light input from the input terminal 1 is input to the multiplexer 5 via the optical coupler 3 and the optical isolator 4-1. The multiplexer 5 includes an excitation light source 6
The multiplexed pump light and the wavelength-multiplexed signal light output from the optical fiber are input to the rare-earth-doped fiber 7. The output light of the rare earth-doped fiber 7 is output from the output terminal 2 via the optical isolator 4-2 and the optical variable attenuator 13. The light partially branched by the optical coupler 3 is input to an optical weighting circuit 11 that weights the optical power of each wavelength at an attenuation rate having a predetermined wavelength dependency. The output of the light weighting circuit 11 is input to the light receiving circuit 8 and is converted into an electric signal. The control circuit 14 controls the pump light source 6 so that the output of the light receiving circuit 8 has a predetermined pump light power, and controls the attenuation of the variable optical attenuator 13.

FIG. 8 shows the output spectrum of a fluoride-based erbium-doped fiber optical amplifier. The wavelengths λ _{1 to} λ ₈ of the input wavelength multiplexed signal light are 1532 nm to 1560 nm.
At intervals of 4 nm. Signal light power per wave is -18
dBm. In the example shown here, compared to the silica-based erbium-doped fiber optical amplifier, 1530 nm to 1560 nm
It can be seen that the gain deviation is small over the range.

FIG. 9 shows the gain of each wavelength when the input power of all wavelengths is reduced uniformly by the fluoride-base erbium-doped fiber optical amplifier and the power of the pumping light is adjusted so that the gain deviation becomes constant. Indicates a change. As an initial condition, the input power per wave is -18dBm, while the input power is 0.58, 1.25, 2.04, 3.01, 4.26, 6.02, 9.03dB.
This is a case where the number is decreased by one. In the erbium-doped fiber optical amplifier based on the fluoride, when the input power is reduced and the pumping light power is adjusted so that the gain deviation becomes constant, the gain is found to increase uniformly at each wavelength. FIG. 10 shows the average values of the input power and the gain change obtained from FIG.

As described above, in the rare earth-doped fiber optical amplifier based on the fluoride, when the input power of the wavelength multiplexed signal light changes, the pumping light power is controlled so that the gain deviation becomes constant at each wavelength. The gain at all wavelengths changes uniformly. Therefore, the gain and the gain deviation can be controlled to be constant by giving the attenuation amount proportional to the gain change shown in FIG. 10 by the optical variable attenuator 13.

FIG. 11 shows a change in gain at each wavelength when the number of multiplexed wavelengths changes in the fifth embodiment. In the figure, the crosses indicate changes in the gains of the remaining signal lights when the input of the signal lights of the wavelengths λ ₁ , λ ₅ , λ ₈ having the larger gain among the wavelength multiplexed signal lights is eliminated. The symbol ◇ indicates a change in the gain of the remaining signal light when the input of the signal light of the wavelength λ ₂ , λ ₃ , λ ₄ having the smaller gain of the wavelength multiplexed signal light is eliminated. For comparison, the change in gain at each wavelength in the configuration without the optical weighting circuit is also indicated by □ and 印. As shown in the figure, by using an optical weighting circuit, ± 0.
It can be seen that the gain change of 5 dB or more was suppressed to within 0.1 dB.

As described above, the input signal light is weighted by the optical weighting circuit 11, the output thereof is controlled so as to have a predetermined pumping light power, and the attenuation which is set in advance for all the input powers is controlled. By controlling the variable optical attenuator so that the following equation is obtained, there is no change in the gain of each wavelength with respect to a change in the number of wavelength multiplexes, and it can be seen that the gain can be controlled to be constant.

The variable optical attenuator 1 according to the present embodiment
The third embodiment is applied to the first embodiment (FIG. 1) corresponding to claim 2, but the second embodiment (FIG. 3) corresponding to claim 3 or the third embodiment corresponding to claim 4. The same applies to the fourth embodiment (FIGS. 5 and 6). In addition, by cascading a plurality of optical amplifiers of the above-described embodiments in an arbitrary combination, the gain of each wavelength is controlled to be constant with respect to a change in the number of multiplexed wavelengths while maintaining a predetermined gain. can do.

[0072]

As described above, the optical amplifier of the present invention can obtain the input / output power of the wavelength division multiplexed signal light which is not affected by the gain deviation by using the optical weighting circuit, and can change the total input power. The gain of the signal light of each wavelength can be controlled to be constant with respect to both the change in the wavelength and the number of multiplexed wavelengths. Also, since such an optical weighting circuit can be realized by a passive circuit, the configuration is simple and no complicated control is required.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the optical amplifier of the present invention.

FIG. 2 is a diagram illustrating a change in gain at each wavelength when the number of multiplexed wavelengths changes in the first embodiment.

FIG. 3 is a block diagram showing a second embodiment of the optical amplifier of the present invention.

FIG. 4 is a diagram showing a change in gain of each wavelength when the number of wavelength multiplexes changes in the second embodiment.

FIG. 5 is a block diagram showing a third embodiment of the optical amplifier according to the present invention.

FIG. 6 is a block diagram showing a fourth embodiment of the optical amplifier according to the present invention.

FIG. 7 is a block diagram showing a fifth embodiment of the optical amplifier according to the present invention.

FIG. 8 is a diagram showing an output spectrum of a fluoride-based erbium-doped fiber optical amplifier.

FIG. 9 shows a change in gain at each wavelength when the input power at all wavelengths is uniformly reduced by the fluoride-based erbium-doped fiber optical amplifier and the pumping light power is adjusted so that the gain deviation is constant. FIG.

FIG. 10 is a view showing an average value of input power and gain change obtained from FIG. 9;

FIG. 11 is a diagram showing a change in gain of each wavelength when the number of multiplexed wavelengths changes in the fifth embodiment.

FIG. 12 is a block diagram showing a configuration example of a conventional optical amplifier.

FIG. 13 is a diagram illustrating a calculation example of gain for each wavelength when wavelength-division multiplexed signal light is input to a conventional optical amplifier.

14 is an optical amplifier having a gain characteristic shown in FIG.
FIG. 7 is a diagram illustrating a change in gain at each wavelength when the number of wavelength multiplexes changes.

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 output terminal 3 optical coupler 4 optical isolator 5 multiplexer 6 excitation light source 7 rare earth doped fiber 8 light receiving circuit 9 logarithmic amplifier 10, 12, 14 control circuit 11 optical weighting circuit 13 optical variable attenuator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication H04J 14/02

Claims (5)

[Claims] An optical fiber doped with a rare earth element; an excitation light source for generating an excitation light for exciting the optical fiber; an excitation light input for multiplexing the wavelength multiplexed signal light and the excitation light and inputting the optical fiber to the optical fiber Monitoring means for weighting and monitoring the power of the wavelength-division multiplexed signal light with an attenuation factor having a predetermined wavelength dependency, wherein the optical amplifier outputs the wavelength-division multiplexed signal light amplified from the optical fiber. An optical amplifier comprising: a control unit that controls the power of the pump light according to an output of the monitoring unit. 2. The optical amplifier according to claim 1, wherein the monitoring unit is configured to branch a part of the wavelength-division multiplexed signal light input to the rare-earth-doped optical fiber; Has a loss characteristic inversely proportional to the characteristic,
An input including an optical weighting circuit for weighting the wavelength multiplexed signal light branched by the branching unit, and a light receiving circuit for outputting an electric signal proportional to the total optical power of the wavelength multiplexed signal light output from the optical weighting circuit; An optical amplifier, comprising: a signal light power monitoring circuit; and a control means, comprising: means for controlling an excitation light source so that an output of the light receiving circuit has a predetermined excitation light power. 3. The optical amplifier according to claim 1, wherein the monitoring means comprises: a branching means for branching a part of the wavelength multiplexed signal light input to the rare-earth-doped optical fiber; and a gain-wavelength of the optical fiber. An optical weighting circuit having a loss characteristic inversely proportional to the characteristic, and weighting the wavelength division multiplexed signal light branched by the branching means; An input signal light power monitoring circuit including a light receiving circuit that outputs a signal, a branching unit that branches a part of the wavelength multiplexed signal light output from the optical fiber, and a wavelength multiplexed signal light branched by the branching unit. An output signal light power monitoring circuit including a light receiving circuit that outputs an electric signal proportional to the total optical power; and control means for controlling the output of the input signal light power monitoring circuit and the output signal light power. A logarithmic amplifier for outputting a logarithmic difference between the output of the monitoring circuit, an optical amplifier output of the logarithmic amplifier is characterized by comprising a means for controlling the excitation light power to be constant. 4. The optical amplifier according to claim 1, wherein the monitoring unit is a branching unit that branches a part of the wavelength multiplexed signal light input to the rare-earth-doped optical fiber, and the branching unit branches the branching unit. An input signal light power monitoring circuit including a light receiving circuit that outputs an electric signal proportional to the total optical power of the wavelength multiplexed signal light, and a branching unit that branches a part of the wavelength multiplexed signal light output from the optical fiber, An optical weighting circuit that has a loss characteristic inversely proportional to the gain-wavelength characteristic of the optical fiber and weights the wavelength multiplexed signal light either before or after the branching unit; and a wavelength multiplexing circuit output from the optical weighting circuit. An output signal light power monitoring circuit including a light receiving circuit that outputs an electric signal proportional to the total optical power of the signal light, wherein the control means includes an output of the input signal light power monitoring circuit and the output. A logarithmic amplifier for outputting a logarithmic difference between the output of the issue optical power monitoring circuit, an optical amplifier output of the logarithmic amplifier is characterized by comprising a means for controlling the excitation light power to be constant. 5. The optical amplifier according to claim 2, wherein an optical variable attenuator is connected to an output stage of the rare-earth-doped optical fiber, and an output of the input signal optical power monitoring circuit is provided. An optical amplifier comprising means for controlling the variable optical attenuator so that the attenuation becomes a preset attenuation amount.